Microstructural evolution of Au–35 mass% Ag–15 mass% Cu alloy during ageing at 415 °C

Abstract

The thermal analyses of Au–35 mass% Ag–15 mass% Cu alloy show different phase transformations (order-disorder transition and precipitation reaction) which have not been well identified before. It is known that the physical properties of this alloy depend on the crystalline and microstructural changes. Thus, to identify all phases optical microscopy, X-ray diffraction (XRD), differential scanning calorimetry, scanning electron microscopy and high-resolution transmission electron microscopy are used. Moreover, the crystalline structure of the formed phases has been followed by XRD, elemental X-ray mapping and electronic diffraction. These techniques show for the first time, the simultaneous occurrence of discontinuous precipitation and disorder-order transition during ageing at the same temperature 415 °C. Indeed, cells with lamellae of Ag-rich α1 and Cu-rich α2 solid solutions and AuCu3-ordered phase are, respectively, formed at grain boundaries by discontinuous decomposition of Ag-rich α1 supersaturated solid solution and in the grain interior of the α0 parent Au-rich phase. The presence of a third phase—Cu-rich α2 solid solution—is revealed in the ternary Au–Ag–Cu phase diagram at 400 °C isothermal section.